I think I'd like to change out the smaller windows that have three panes in them. The center one is stationary and the sides swing out, they're not very practical if you want air circulation. As far as removing the diamond grids, I do plan to take them out but I still want some visual interest aside from just a plain window. I'm just not sure what kind would work best and how to handle the large front window. Also, I'm not great with window terminology so hopefully I'm getting my point across.

I would use whatever sealer the tile manufacturer or the company from whom you purchased the tile, recommends.

It sounds like you may be mounting the blinds in the improper location. The depth should be the same at the top and bottom of the opening unless the blinds are being mounted in the actual "window" portion of the opening. The blinds are not supposed to be mounted in the area where the window slides up. They are to be mounted in front of that. The photos are hard to tell what the depth is like but if the mounting depth shallow, you may want to consider doing an outside mount, meaning the blind/shade would mount on the trim work.

That TV is so high up that I think you'll find it very uncomfortable. Ideally, you should be able to look at the TV straight on without tilting your head up or down (otherwise, you can cause major neck issues down the road). Would you be open to putting your TV where that mirrored cabinet currently is? That way, you could lay out your couch and chairs basically the same way it's done in these photos above.